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In settings, I disbled protection, deleted the log files in /var/log/magicspam/ amd enabled protection again.
Also I completely removed the extension from Plesk and reinstalled it again.
However, the problem persists.

I had reinstalled the complete server at September 17th, after which MagicSpam has worked for a couple of days.
The only changes that I can think of that could have affected the working of MagicSpam, is installing more extensions and changing system time zone (to Europe/Amsterdam) and rebooting the server.
At September 25th I deleted the log files in /var/log/magicspam/ as described in an effort to eliminate the problem as described above, without results.

In Plesk > Tools & Settings > Services Management, I restarted Apache, Nginx, PHP-FPM services, and also I restarted MagicSpam through the UI (MagicSpam Protection > Settings > Disable Protection and then Enable Protection), but the statistics for the last 24h are still missing. The time of the incoming mail in the recent log entries is correctly displayed. Do you have other suggestions?

From your post we understand that you were unable to find the provided path and editing /etc/php.ini did not resolve the issue.

To continue troubleshooting this issue, could you please confirm that the /opt/psa/admin/conf/ directory exists?
If the php.ini file does not exist in the /opt/psa/admin/conf/ folder, please create it and add the correct timezone to the file:

Dear MagicSpam Support,
I want to let you know that thanks to your information the problem has been solved now!

Further information:
The /opt/psa/admin/conf/ directory doesn't exist on my system.
However, after another search, I found /usr/local/psa/admin/conf/php.ini
I added the line date.timezone = "Europe/Amsterdam" at the end of this file and I restarted Plesk sw services.
Immediately after that, MagicSpam was showing the statistics for last 24 hrs again and is working as expected now.
Thanks again for your oustanding support!